The present invention relates to an electrical circuit and method for generating an adaptive discriminating voltage used for demodulation of data signals into digital signals at a receiving side, and particularly for generating such a voltage used for demodulation of data signals of which relatively low frequencies are cut off during modulation, transmission or reception thereof.
The transmission of digital signals in a radio transmission line or channel has lately attracted considerable attention. There have been many different ways of modulating digital signals into data signals adapted for a radio transmission. In a mobile radio transmission system which transmits data signals through individual channels each, respectively assigned to a corresponding carrier wave this is, which may be called "single channel per carrier wave transmission", there are, for example, a modulation method for directly modulating binary digital signals into frequency modulation (FM) data signals and a modulation method for modulating data signals, for example, digital signals attenuated by a low-pass filter such as a gaussian filter, into FM data signals. The latter modulation method is called "Gaussian Filter Manipulated Minimum Shift Keying" (GMSK) modulation and requires a relatively narrower transmission band. There is also a method for modulating data signals obtained from binary digital signal and having tetrad levels into FM data signals.
In the above mentioned system, because of the presence of direct current (D.C.) cut-off circuits, it is difficult to faithfully transmit the D.C. or low frequency components of such FM data signals. As a consequence, owing to the variation of center voltage of the data signals at a receiving side, it is also difficult to faithfully demodulate such data signals into digital signals without the D.C. components of the data signals. In order to improve such faithfulness, Yoshiki HIGO et al., in "Considerations on Quantized Feedback DC Restoration for a High Speed Digital Repeater", Report of Technical group, CS Vol. 75 No. 2, P 93-94, Institution of Electric and Communication Engineers of Japan (July 24, 1975), has proposed an approach that reproduces D.C. components of data signals at a receiving side. The above approach, however, requires a special automatic gain control circuit at the receiving side which can linearly control the amplitude of the received data signals. Such a circuit is requested because a demodulator according to this approach operates effectively only on condition that the amplitude of demodulated digital signals is identical to that of the received digital signals. As a result, this approach also causes a complication in the design of the demodulator and, further, can not be applied to the system of demodulating data signals composed of more that two levels.